Posix systems record information on processes using a 16-bit integer. The
lower bits record the process status (stopped, exited, signaled) and the
upper bits possibly contain additional information (for example the
program's return code in the case of exited processes). Pre Ruby 1.8, these
bits were exposed directly to the Ruby program. Ruby now encapsulates these
in a Process::Status object. To maximize compatibility,
however, these objects retain a bit-oriented interface. In the descriptions
that follow, when we talk about the integer value of stat, we're
referring to this 16 bit value.